Polythioethers of high molecular weight are a widely investigated useful class of polymers. Most high molecular weight polythioethers have been prepared by the anionic polymerization of episulfides. With the exception of the highly crystalline polyethylene-sulfide, these polymers are useful as elastomers. These elastomers are generally copolymers. Minor amounts of olefinic unsaturation are incorporated into the polymer chain as crosslinking sites. This unsaturation is derived from an unsaturated episulfide comonomer.
The successful crosslinking of high molecular weight unsaturated episulfide copolymers initiated further research to find novel types of reactive polythioethers which can be crosslinked to rubbery networks. Such rubbers are of high interest mainly because their high sulfur content results in very good hydrocarbon solvent resistance and excellent aging characteristics.
The preparation of some terminally difunctional polythioethers by the addition of dithiols to diolefinic hydrocarbons has been known for some time. For example, Marvel and Chambers, J. Am. Chem. Soc. 70,999 (1948) and Marvel and Cripps, J. Polymer Sci. 8, 313 (1952) reported the reaction of dithiols with conjugated dienes such as butadiene and with dienes having isolated double bonds. However, such reactions proceeded at very slow rates and often resulted in unsaturated thioethers.
Numerous polythioethers were prepared by the ring opening of episulfides. The terminal groups of these polymers were generally not disclosed. However, they can be inferred on the basis of the initiators used since the mechanism of such ring opening reactions is known. It can be safely assumed that these polythioether preparations did not result in diterminally thiol and/or vinyl sulfide functional polymers. Inherent to the episulfide ring opening catalysts used, generally polythioethers containing only one thiol group were formed. The other end group derived from the initiator was usually a nonreactive hydrocarbon end group. For example, Boileau produced naphthyl terminated polythioether thiols by using sodium naphthyl as an initiator as described in the journal Compt. rend. (Paris) 254, 2774 (1962). Another example is given by the ethyl terminated polythioether thiol polymers which were produced by zinc diethyl initiator as disclosed in U.S. Pat. No. 3,222,326, incorporated herein by reference.
The use of amines as initiators of episulfide polymerization resulted in polyfunctional polythioethers. However, their use also resulted in the incorporation of nitrogen into the polymer (see, for example, U.S. Pat. No. 3,325,456).
Some polythioethers having reactive thiol end groups are disclosed in U.S. Pat. No. 3,337,487, incorporated herein by reference. These polythioethers result from the cleavage of nonfunctional very high molecular weight polymers derived by episulfide polymerization. Since episulfides such as ethylene and propylene episulfide cannot be copolymerized in an alternating manner, the method disclosed results in thiol terminated copolymers having a random structure.